Packaging of ultraviolet (UV) transmitters, such as Light Emitting Diodes (LEDs), generally involves ‘flip-chip’ bonding the LED to another chip to allow light extraction through the underlying substrate. Typically, flip-chip bonding is one type of mounting used for semiconductor devices, such as IC chips, to mount chips together or in a package. In this process solder beads are deposited on one of the chip's electrical pads, and then the individual LED die, or ideally the entire wafer of LED die, is mounted upside down on the package, or receiving chip. This leaves the LED chip's electrical connections facing down onto the package, while the back side of the die faces up.
Issues arise in the packaging and mounting processes of LEDs with regard to properly heat sinking the LED region for heat management, as well as making isolated but robust connections to the p and n contacts of the LED. Depositing thick metal over the p and n contacts, on the order of micrometers (microns), and an electrical passivation layer between them helps alleviate both of these problems. The thick metal pad acts as a heat sink, while ensuring that the contacts to the isolated p and n regions make the necessary electrical connections. The electrical passivation layer is essential to preventing the two contacts from shorting together during the bonding process.
Electroplating generally forms the thick metal pads. During the typical electroplating process for the thick metal pads, extra effort ensures that the ground plane layer has electrical contact with the electroplating seed layer. The extra effort generally involves a photolithography step with an additional mask layer, over etching of a passivation layer, or a deliberate misalignment during a photolithography process.
Each extra step adds complexity and cost to the manufacturing process that in turn affects the cost of the resulting chip or chips. Removal of any of the processes within the manufacturing process would lower the complexity and decrease the cost of manufacturing the resulting packaged LEDs on the chips.